The contacts are shown as they would be for a normal 'real world' relay, that is, they are shown in a state of no power (not energized) applied. As you point out, the description of the EXTERNAL circuit sensors says that thes inputs will be turned on when there is no fluid covering the sensor. In that case the external sensor will turn ON, the PLC's input will be turned ON and the state of the contacts show will then be true.
This is NOT shown while runnning by changing the normally open (NO) symbol ( --| |-- ) to normally closed (NC) ( --|\|-- ). In most PLC's it is shown by highlighting the contact with a contrasting background. This highlight means the contact has now switched from the state of the graphic symbol (its OFF state) to the opposite one. So in this exercise, imagine this highlight on the input sensor symbol expressing a NOW closed state.
